The NHS has been accused of covering up “worrying and persistently high” death rates in Welsh hospitals and ignoring calls to investigate by England’s most senior doctor.

The disclosure on Wednesday night provoked a political row, with Labour accused of presiding over serious failures at NHS hospitals in Wales, where it has held power since devolution. Emails seen by The Telegraph show that Prof Sir Bruce Keogh, the NHS England medical director, called for an investigation three months ago.

MPs on Wednesday night raised fears that “deeply disturbing” failings were being allowed to continue.

Jeremy Hunt, the Health Secretary, said the failure was a “betrayal of patients” and accused the NHS in Labour-controlled Wales of not learning the lessons of Mid-Staffs, where hundreds of patients died amid failings in care.

The six hospitals, all with persistently high death rates, were brought to the attention of Sir Bruce by Ann Clwyd MP last year. He alerted Chris Jones, his Welsh counterpart. In an email Sir Bruce said the statistics were “worrying” and offered assistance with an investigation, having recently held such an inquiry into 14 hospitals in England.

No investigation was launched and he received no reply. Labour’s first minister had rejected calls from MPs for an examination of death rates the previous month, saying it was unnecessary.

In his email, Sir Bruce tells Chris Jones, medical director of NHS Wales: “There are six hospitals with persistently high mortality which warrrant investigating.”

“Waiting times in A&E are considerably worse than in England, but the real concern is around prolongued (>6 weeks) waiting times for diagnostics which, of course, translates to delayed treatment.”

The correspondence states that Wales has “considerably worse” waiting times in Accident & Emergency departments, compared with England, and “worrying” data showing prolongued waiting times for tests and life-saving treatment.

It discloses that half of patients in the country are waiting at least six weeks for MRI scans and tests for bowel cancer. Forty per cent waited as long for tests to check for heart failure and basic ultrasound scans, the correspondence

In England, less than one per cent of patients wait so long for any such diagnostic tests, the letter states.

Sir Bruce describes the data as “worrying” and offers to assist with a review of death rates, warning his counterpart: “I am worried about the broader political implications of the data that has been shared with me by Ann Clwyd.”

The information provided by Mrs Clwyd showed high death rates at University Hospital Wales, in Cardiff; Royal Gwent in Newport; Princess of Wales in Bridgend; Royal Glamorgan in Llantristant and three community hospitals.

Sir Bruce received no reply to his email, and no investigation was ordered.

Charlotte Leslie, a Conservative member of the Commons health committee, who obtained the correspondence, said on Wednesday night: “After all the fatal cover-ups in Mid Staffs, which cost countless lives under Labour, there is still a corner of the NHS where they preside – and yet again we see cover-up and inaction.”

Earlier this month, Jeremy Hunt, the Health Secretary, said it was “a tragedy” that the NHS in Wales had failed to adopt measures being introduced in England in the wake of Mid-Staffs- including investigations of hospitals with high death rates.

Mrs Clywd, Labour MP for Cynon Valley, in Wales, said: “The situation is deeply disturbing. I have been calling for an inquiry into death rates for some time because several of the figures are extremely worrying.

“People in Wales ought to be able to have as good care as anywhere else, but instead here are people dying on hospital waiting lists, and suffering appalling failings in care,” she added.

Mrs Clywd, Labour MP for Cynon Valley, in Wales, previously led a government review of the NHS complaints system, after her husband Owen suffered poor care at University Hospital Wales.

The latest published figures show that hospital has mortality rates 28 per cent above average in Wales.

A Welsh Government spokesman said Sir Bruce had stated that he could not reach conclusions about the death rates and added: “If legitimate concerns are raised about particular aspects of care at specific hospitals, we have acted quickly and ordered independent reviews.” A spokesman for The Princess of Wales Hospital said it would not be appropriate to comment on calls for an investigation, as this was a matter for the Welsh government. He said an NHS review found the trust was a "clinically-led, data-driven" organisation and that its findings would be published soon.

A spokesman for Royal Glamorgan Hospital said they had introduced a system of reviewing all deaths to check whether there had been failures in care, and that latest mortality figures showed a “significant improvement”.